Function build 

pub fn build<F>(domain_suffix: &str, f: F) -> Vec<u8> 
where
    F: FnOnce(&mut Builder),

Convenience: build a payload by chaining mutations on a Builder.

let bytes = build("ref-update", |b| {
    b.field_bytes(&repo_key);
    b.field_str("refs/heads/main");
    b.field_bytes(&commit_hash);
    b.field_u64(update_seq);
    b.field_u64(auth_epoch);
});